The present invention relates to a method of producing a chemical vapor deposition of zinc borosilicate glass which is suited for use as a surface protection film for semiconductor devices, insulation layers for integrated circuits and thin film capacitors.
As one of methods of protecting the surfaces of the semiconductor devices it is known to form a glass film. In the formation of such a glass film zinc borosilicate glass powders are deposited on the surface of the semiconductor device to be protected by a centrifugal sedimentation method, a slurry process or the like and subsequently subjected to a fusing treatment. Accordingly, the resulted glass film is usually relatively thick, and for this reason the use of the glass film is exclusively restricted to the application such as a surface protection film for diodes of a high breakdown voltage, stacked diodes or for the mesa portion of a semiconductor device of mesa-type or the like but it is difficult to use as the surface protection film for the integrated circuit devices or for the semiconductor device of planar type.
As attempt to make the thin glass film produced in the above described manner, it will be encountered many difficulties. For example, when the glass film is to be formed starting from the powder of the glass, roughness will be produced in the finished glass surface. Further, hollow spaces among the particles of the glass powders will tend to remain as voids in the formed glass layer. Under these conditions, the thickness of the glass film is so selected as to be greater than the diameter of such voids, which necessarily imposes limitation in the attempt to make the thinner glass film. When the glass film having a relatively large thickness is etched by using a photo-resist, it is impossible to attain a desired precision of the pattern formed by etching, which in turm renders the formation of a high density pattern impractical. Furthermore, since voids remain in the formed film, it is difficult to obtain a relatively thin film having a relatively large dielectric strength. A thick film evaded from such drawbacks provides also disadvantage that lead wires which are to be connected to electrodes of a device along the surface of such a thick glass film are easily broken at the edge portion of the glass film.
If such a thick glass film of a conventional type as described above is used for an integrated circuit when it is desired to perform a high integration density, unavoidable etching error due to a large thickness of the film will necessarily involve a reduced precision in the produced pattern of the circuit configuration.
With a view of making the film thinner, thereby to enhance the integration density, it is known to use a silicon oxide film as the protection film. However, a single layer of such protection film is insufficient for attaining a wanted moisture resistance as well as dielectric strength. As an attempt to increase the dielectric strength and at the same time decrease the surface charge storage, it is also known to form a phosphorous glass coating over the silicon oxide layer. When the precision of pattern formed by etching in the device using the phosphorous glass is to be improved, content of phosphorus in the glass material has to be increased. However, high content of phosphorus component in the glass composition will disadvantageously result in a decreased moisture resistance. Consequently, the single layer protection film of the conventional type can not assure an adequate protection function.
It has been also proposed a method of producing a protection glass film according to which vapor produced by evaporating a halogen compound of metals is fed into a reaction chamber by means of a carrier gas thereby to deposit the metal elements on the surface of a heated substrate disposed in the reaction chamber in a form of a pure metal deposition or in a partially oxidized deposition, which is subsequently subjected to a heating process in the presence of an oxidizing atmosphere. According to another known method, vapor produced by evaporating the above-mentioned compound is successively fed to the reaction chamber together with oxidizing gas thereby to form deposition of oxides of evaporated metals on the substrate surface in a form of a multi-layer structure, which is thereafter fused together under the heating to form a glass film.
However, in the case of the first method in which the deposited metals or imperfect oxides thereof undergo subsequently oxidation treatment, it is difficult to make all the deposits glassy or to be vitrified, as a result of which a difference in thermal expansion may occur between the surface and the inner regions of the glass film to give rise to delamination in the vicinity of the interface between the film layer and the substrate, or otherwise nonhomogeneous formation of the film having non-uniform distribution of composition in the transversal direction of the film. On the other hand, in the case of method in which the successively formed multi-layer of metal oxides is subsequently heated to be fused together, a high temperature is required for vitrification, not to speak of adverse effects of the difference in the thermal expansion coefficients among the various metal oxide layers. Accordingly, this method can not be applied to certain types object.
Method of depositing a glass of a three-component series on a semiconductor device through a chemical vapor deposition process is disclosed in U.S. Pat. No. 3,481,781 to W. Kern. The attempt to form a homogeneous thin glass film through the chemical vapor deposition process has been encountered with various problems to be solved. For examples, difficulties lie in controlling the glass composition at predetermined values, increasing adhesiveness between the glass film and a substrate, enhancing the growing rate of deposited glass, assuring a safety in the working environment, forming the film of a predetermined uniform thickness over a wide area, etc.
The inventors of the present application have found that the above described various problems can be substantially solved by selecting predetermined types of starting materials, using them in selected combination and correspondingly modifying the nozzle structures of conduits used for supplying the raw materials. For example, when diborane (B.sub.2 H.sub.6) and silane (SiH.sub.4) are employed as raw materials to form a glass of combined boron oxide and silicon oxide, the vitrification reaction will be accompanied with undesirable auxiliary reactions which affect adversely the formation of the wanted homogeneous glass, not to speak of the troublesome handling of diborane due to the deadly poisonousness thereof. The occurrence of such secondary reactions depends on the presence or absence of other components and in turn affects the reaction temperature as well as the chemical vapor growth rate of glass. In order to suppress the secondary reactions at minimum and at the same time permitting a homogeneous vitrification reaction to occur over a wide area, it is necessary to use injection nozzles of specifically devised structures for the conduits to supply the raw material gases. The term "reaction temperature" as herein used means a temperature of a substrate on which the chemical vapor deposition can take place.
The composition and components of the glass for vitrification of the surface of silicon semiconductor device are restricted to 58 to 80% by weight of zinc oxide (ZnO), 14 to 30% by weight of boron oxide (B.sub.2 O.sub.3), 5 to 15% by weight of silicon oxide (SiO.sub.2), and less than 10% by weight of rest components in view of the melting temperatures and the thermal expansion coefficients of the composition components. Hereinafter, the composition will be given in term of the percent by weight so far as it is not otherwise specified. 67 to 75% of ZnO, 15 to 22% of B.sub.2 O.sub.3, and 7.5 to 13% of SiO.sub.2 are a preferred range of the composition according to the invention. In this conjunction, the thermal expansion coefficient should be in the range of 41 .times. 10.sup.-7 to 55 .times. 10.sup.-7 /.degree. C at the temperature of 25 to 300.degree. C and preferably of 41 .times. 10.sup.-7 to 44 .times. 10.sup.-7 /.degree. C.